The production of oil and gas from subterranean reservoirs frequently results in the build-up of scale within wellbore pipe. Scale of any thickness impedes the flow of oil and gas through the pipe, lowering oil and gas production rates. Furthermore, thick scale accumulations prevent the movement of tools within the pipe. It is, therefore, desirable to prevent scale from forming. When efforts to this end are unsuccessful, however, the pipe must often be mechanically cleaned.
Scale is frequently created when reservoir liquids transport dissolved sulfates through wellbore pipe. As the liquids approach the earth's surface, reductions in temperature and pressure cause the sulfates to precipitate out of solution and collect on the inside of the pipe. Scale deposits can vary in consistency from a thick sludge to a brittle solid, making their removal difficult. Further complicating the removal of scale from oilfield pipe is the fact that the scale is often contaminated with radioactive compounds.
Radioactive scale which has accumulated on oilfield pipe is considered to be a naturally occurring radioactive material (NORM). NORM removed from tubing can vary greatly in terms of its radioactivity. Some NORM samples have been found to possess a level of radioactivity that is roughly 100,000 times higher than typical soil. Although the NORM found in oilfield tubing is generally considered to be non-hazardous, it is desirable to minimize human contact with it.
Cleaning oilfield pipe can expose workers to NORM that may pose health risks. Inadvertent inhalation and ingestion of NORM for prolonged periods can increase the risk of cancer and bone abnormalities. Radioactivity from NORM brought close to a human body can also penetrate skin causing cellular damage. A safe limit for exposure to NORM is unknown and may vary from person to person.
The cleaning of oilfield pipe generally involves the insertion of a tool-carrying lance into the pipe. Once inside the pipe, the tool engages the scale. By the rotation of the tool or the pipe, the scale is typically scraped from the interior wall of the tubing. The dislodged scale particles are flushed from the tubing by a stream of water or air channeled through the lance. Afterward, the scale particles are collected for safe disposal.
Since the cleaning of scale from oilfield pipe often results in the concentration of NORM, it is especially important to prevent its uncontrolled spreading. Unfortunately, the available equipment for cleaning oilfield pipe has been known to create a “toxic dust” that can be blown by the wind into surrounding neighborhoods. Furthermore, this equipment is not especially good at removing scale having great hardness from the interior of pipe.